1. Field of the Invention
The present invention relates to a method of processing a substrate to be processed, e.g., a semiconductor wafer, by using a plasma.
2. Description of the Related Art
In a semiconductor device manufacturing process, various kinds of plasma processes such as plasma etching are performed with respect to a semiconductor wafer in a process chamber in a vacuum atmosphere. An electrostatic chuck having a first electrode interposed between two insulating layers is disposed on the wafer table in the process chamber in order to fix the wafer on the table. A second electrode is formed apart from the first electrode.
In a plasma etching apparatus, while the process chamber as the second electrode is grounded, RF power is supplied to the wafer table to generate a plasma around the wafer placed on the electrostatic chuck. The wafer is grounded through the plasma. When a high voltage is applied to the first electrode of the electrostatic chuck, positive and negative charges are respectively produced in the wafer and the first electrode, so that the wafer is attracted/held to/on the wafer table by the Coulomb force acting between the wafer and the first electrode. Etching is performed in this state. When the etching is completed, the supply of the RF power to the wafer table and the application of the high voltage to the first electrode are stopped. Subsequently, the processed wafer is unloaded, and the next wafer is loaded.
It is empirically known that the attractive force of the electrostatic chuck is reduced as a plasma process is repeated by the above-described method. This phenomenon is especially conspicuous when the insulating layers of the electrostatic chuck are made of polyimide. Although an exact cause for this phenomenon is not identified yet, a likely cause is that residual charges are accumulated on the support surface of the electrostatic chuck, and the Coulomb force acting between a wafer and the first electrode is decreased by the residual charges.
Furthermore, in the plasma etching apparatus, a temperature adjusting portion such as a cooling portion is arranged in the wafer table to adjust the temperature of a wafer. In addition, the space between the wafer and the electrostatic chuck is filled with a gas serving as a heat transfer medium, e.g., He, in order to improve the heat transfer rate between the wafer and the electrostatic chuck. Filling of the heat transfer gas must be performed after the wafer is fixed by the electrostatic chuck. For this reason, in a conventional method, filling of a heat transfer gas is performed after the following sequence of steps: placing a wafer on the electrostatic chuck; and applying a voltage to the electrostatic chuck to generate a plasma, thereby causing the wafer to be grounded through the plasma and fixed on the table. In this conventional sequence, however, an increase in the temperature of the wafer upon generation of the plasma cannot be adjusted until filling of the heat transfer gas is completed. Therefore, the wafer may be adversely affected by such an increase in temperature. In addition, the starting time required to start an etching process is prolonged.